Polyimide resin has excellent heat resistance, an excellent electrical insulating property, excellent chemical resistance, and a mechanical property, and therefore is used for electric and electronic purposes. For example, the polyimide resin is used as materials of (i) an isolation film and a protective coating agent provided on a semiconductor device, (ii) a surface protective material and a base material resin for a flexible circuit substrate, an integrated circuit, and the like, and (iii) an interlayer insulating film and a protective film for a fine circuit. Particularly, in a case where the polyimide resin is used as a coating material, the polyimide resin is used as (a) a protection material made by applying an adhesive agent on a shaped product such as a polyimide film, (b) a liquid-form polyimide resin solution, and the like.
The polyimide resin solution is roughly classified into two types: one type is a polyamide acid solution, which is a solution of a polyimide resin precursor; and the other type is a polyimide solution that contains polyimide resin soluble in an organic solvent. However, the polyamide acid solution and the polyimide solution are polymer solutions of a high molecular weight polymer and have a large molecular weight and low solubility to a solvent. This makes it difficult to prepare these solutions to have a high solute concentration, and therefore it is necessary to evaporate a large amount of the solvent in order to form a coating film, for example. This renders productivity of the coating film poor. Further, in a case of the solution of the polyimide resin precursor, it is necessary to imidize a coating film, which is formed by applying the solution. At this time, the imidization should be carried out at a heat temperature of more than 300° C. However, in a case where the solution is used, for example, as a protection agent for a flexible substrate or the like, or as an adhesive agent to be applied on a shaped product, since wiring materials cannot endure a high temperature, it is necessary to use resin that is curable at a low temperature (not more than 250° C.) so as not to cause deterioration of wiring lines.
In relation to a technique for providing such a polyimide resin solution, there have been proposed a high-concentration and low-viscosity polyimide precursor solution in which a diamine and either a tetracarboxylic acid or its diester acid derivative are dissolved (see Patent Literatures 1 to 4, for example).
Further, there has been also proposed a high-concentration and low-viscosity polyimide precursor solution in which a diamine and a carboxylic acid having an amide bond in its structure are dissolved (see Patent Literatures 5 to 7, for example).
Moreover, there has been reported a photosensitive resin composition or a plasma etching resist that uses an imide siloxane oligomer having a half-esterified terminal (see Patent Literatures 8 to 11, for example).